1. Field of the Invention
The present invention relates generally to pressure sensors and, more particularly, to sensors used to determine the pressure of an operating device, such as a screw-type refrigeration compressor. In its most preferred embodiment, the present invention is utilized with refrigeration compressors to determine whether the compressor, after stat-up, is operating in a positive or negative mode.
2. Description of the Prior Art
Differential pressure sensors are known. In commonly owned U.S. Pat. No. 5,024,294, issued Jun. 18, 1991, Van Fossen, et al. describe a "Differential Pressure Transducer" which includes a housing forming a high pressure chamber, with a nozzle mounted on the housing at one end of the chamber, a diaphragm mounted in the chamber to form a low pressure chamber in the nozzle, a spring retainer in the high pressure chamber, a spindle connected to the diaphragm, a tension spring connecting the spindle to the spring retainer, a circular magnet mounted on the spindle, and a transducer mounted on the housing to sense the position of the magnet. In this device, only positive pressures could be sensed, i.e. pressures exceeding 0 psig. While such sensor has been acceptable in its field of use, the sensor is incapable of determining negative pressures, such as negative pressures occurring when the compressor rotates in the wrong direction. It would be advantageous to be able to determine the direction of rotation, because when a screw compressor is driven by a three-phase electric motor, it is possible to start the compressor in either direction. Although typically this will not be detrimental to the compressor itself, the refrigeration system with which the compressor is used will not function properly. If a system were designed which could determine whether the pressure was positive or negative at start-up, it would be possible to verify positive pressure and, through a logic circuit after start-up, allow compressor operation to continue. On the other hand, if a negative pressure was sensed in the same time frame, the logic circuit could be designed to shut down the compressor and allow a restart after appropriate corrective action has been taken.
In large, sealed refrigeration compressor systems, it is conventional to monitor lubrication pressure generated by the compressor oil pump to prevent compressor operation when there is insufficient lubrication. The absence of sufficient lubrication can cause premature wear or, in extreme cases, seizing of the compressor. Generally, lubrication pressure is monitored by determining the pressure differential between the crankcase and the output or discharge side of the compressor oil pump. The difficulty in monitoring this differential is due primarily to the variation in crankcase pressures which are dependent on temperature and the type and thermal characteristics of the refrigerant system.
In U.S. Pat. No. 4,551,069, a piston is disposed in a tube separating the interior of the tube into a high pressure portion which communicates with the output of the oil pump and a low pressure portion which communicates with the suction side of the oil pump. The piston is biased by a compression spring to move into engagement with a movable contact arm for a switch which signals loss of pressure. The switch is closed whenever the discharge pressure exceeds the combined force of the compression spring and crankcase pressure. However, this arrangement results in excessive wear of the movable sensor parts due to the continuous movement of the piston with the cyclic pressure fluctuations mentioned above as well as a need for some way to prevent nuisance tripping of the compressor motor if it is to be used to control the energization of the motor as stated in the patent.
In U.S. Pat. No. 4,672,231, a shuttle is mounted within a bore in a cylindrical housing. The shuttle includes a magnet in one end and is biased by a compression spring toward the end of the bore in the cylinder. The magnet is used to operate a reed switch to turn the compressor on or off depending on the pressure differential between the discharge pressure of the oil pump and the combined force of the compression spring and the crankcase pressure. The shuttle is mounted in close contact with the bore in the cylinder to provide a circuitous high pressure flow path through the bore so that the reaction time of the shuttle is delayed in order to minimize the on and off operation of the compressor.
In both of these devices, flow is across the moving part which allows debris to collect on the inlet screen on the high pressure side of the sensor. It should also be noted that both devices use compression springs to bias the pistons and such springs are subject to buckling or uneven closure. This can produce dragging of the magnet within the inner bore causing friction and/or hysteresis.
The problems with these devices, as well as the device shown in the aforementioned Van Fossen, et al. patent, is that negative pressure cannot be sensed accurately using the same device that is used to sense the adequacy of the positive pressure. The provision of a pressure sensor which could accomplish the functions of the sensor of the aforementioned Van Fossen, et al. patent and which would also be able to provide an electrical output indicative of negative pressure, would be a substantial advance in this art.